The invention relates to aerodynamic windows and more particularly to an adjustable-pressure aerodynamic window employing a conical supersonic nozzle.
The aerodynamic window is simply an opening between two regions of different gases, gates at different pressures, or different states of gases where the gas in each region is isolated from the other by aerodynamic means. An example of such a configuration can be a gasdynamic laser test cavity inside a laboratory room with the room being one region and the gas dynamic laser test cavity the other. The gas flow inside the gasdynamic laser test cavity will produce a lower pressure than the room and consequently air will be drawn from the room through any opening between the room and test cavity. To prevent this leakage, aerodynamic means may be employed to balance the pressures across the opening, thus forming an aerodynamic window.
The need for such a device exists when ordinary transparent window material such as plate glass, quartz or plexiglass interferes with the test requirements. For example, the test requirement associated with the previously discussed gasdynamic laser application would be the high energy transmittance of 10.6.mu. laser beam. Beam attenuation and high thermal loading would limit the use of any conventional transparent substance in lasers of high power densities. Absorption of even a small fraction of the incident laser radiation can cause a temperature gradient in the window which results in thickness variations due to thermal expansion and refractive index gradients and stresses which in turn distort the optical beam. This can occur at power levels well below those required to melt or fracture the window.
It should be noted that although the application of an aerodynamic window will be discussed primarily in its use with gas dynamic lasers, other applications in aerodynamic facilities are numerous. For example, the injection of models into a wind tunnel test section could be attained through an aerodynamic window separating the different pressure regions in the test section and outside the tunnel.
An aerodynamic window employs a flowing gas to create a pressure differential which is used to match the laser cavity pressure to atmospheric pressure. Aerodynamic windows may be of two types, those that run from vacuum and those that run from a high pressure gas supply. In general, vacuum windows require a smaller mass flow. However, since a high pressure supply of gas is usually available at high-power laser sites, the pressure aerodynamic windows are more generally employed.
Examples of pressure-driven aerodynamic windows are those described in U.S. Pat. Nos. 3,604,789, 3,617,928 and 3,654,569. In U.S. Pat. No. 3,604,789, issued Sept. 14, 1971 to McLafferty, gas is expanded from atmospheric in a supersonic nozzle where a Prandtl-Meyer expansion flow one on side of the nozzle throat lowers the pressure to that of the cavity. The laser beam passes through the nozzle entrance and an opening on the nozzle wall. In a variation of the design Haussman, in U.S. Pat. No. 3,617,928 issued Nov. 2, 1971, expanded the gas from high pressure to less than an atmosphere and then incorporated the Prandtl-Meyer expansion flow to lower the pressure on the opposite side of the nozzle wall to the cavity pressure. This idea requires a lower mass flow rate. In U.S. Pat. No. 3,654,569, issued Apr. 4, 1972 to Haussmann, gas at supersonic speed flows past a wedge, or isentropic compression surface, to establish a series of oblique shock waves, with the pressure upstream being matched to the cavity pressure and that downstream being equal to the atmospheric pressure.
In these example aerodynamic windows, the contoured nozzles and flow passage are of complicated designs which are expensive to produce and involve precise curvatures to yield the necessary pressure conditions. Each laser cavity must have a specially-designed nozzle and flow passage. Changes in the cavity static pressure resulting from, for example, changes in the gas composition, cannot be readily accommodated and necessitate a totally different aerodynamic window. In addition to those shortcomings the system of McLafferty (U.S. Pat. No. 3,604,789) involves a gas flow that is off-axis from the laser output, resulting in a highly distorted beam as it passes through the pressure gradients.
The conical nozzle aerodynamic window disclosed herein overcomes these and other disadvantages of existing device by utilizing the stagnation pressure drop across a normal shock wave. An axisymmetric nozzle creates a high-Mach-number flow such that the recovery pressure after the normal shock is equal to the cavity pressure. The output beam is normal to all density gradients which minimizes optical distortion. Furthermore, a contoured nozzle is not required and pressure ratios can be easily adjusted.